The invention belongs to the general field of depositing ceramic thermal barrier coatings on hot parts of gas turbines, such as turbojets, for example.
The invention relates more particularly to estimating a thickness of a ceramic thermal barrier coating that is to be deposited by physical vapor deposition on a hot part of a gas turbine, such as a stator guide vane or a rotor wheel blade of a high-pressure turbine.
The invention thus has a preferred, but nonlimiting, application to the field of aviation.
In known manner, while a gas turbine is in operation, its blades are subjected to relatively high temperatures.
In order to avoid the blades deterioration, one solution consists in coating the wall of a blade with a thermal barrier constituted by a ceramic outer layer in order to lower the temperature of the blade. A ceramic commonly used for this purpose is zirconia ZrO2, possibly stabilized with yttrium. Such a ceramic thermal barrier is typically formed by physical vapor deposition (PVD), and more particularly by electron beam assisted PVD (EBPVD).
In the EBPVD technique, the wall of the blade is coated by ceramic vapor condensing thereon in a vacuum enclosure with a partial pressure of an inert or reagent gas. The ceramic vapor is generated by evaporating “target” bars of sintered ceramic that are bombarded by an electron beam.
Only the surface of the blade that is situated facing the surfaces of the ceramic bars is coated with a layer of ceramic by means of that method. Thus, in order to be able to cover the entire profile of the blade, the blade is placed in the vapor during EBPVD deposition on support tooling that is driven with movement in rotation or in oscillation relative to the “target” bars.
At present, the thickness of the ceramic thermal barrier coating that is to be deposited on the wall of the blade is specified by a design office. This specification takes account only of the maximum wall temperature that the blade can accept before deteriorating. However it does not take account of technical constraints associated with actually depositing the coating (e.g. the shape and the movements of the tooling and of the target bars, etc.), such that the coating thickness recommended by the design office cannot always be obtained in practice.
As a result, several tests are generally carried out on real parts in order to act in iterative manner to define support tooling for the blade and to define conditions for EBPVD deposition (e.g. movements of the tooling, exposure times of the hot part to the radiation from the target(s), etc.) making it possible to achieve a coating thickness that is as close as possible to the thickness specified by the design office.
Such tests require both the fabrication and the use of real parts such as blades and various pieces of blade support tooling and also of parts for masking zones of the blade that it is desired not to expose to the radiation from the ceramic bars during EBPVD deposition. Such tests also require the deposits made in this way to be analyzed, including cutting up blades after deposition, measuring deposits on various sections of the blade, and comparing the deposit thicknesses obtained with the specifications from the design office.
In general, at least three tests are needed to achieve the specifications of the design office. It can thus readily be understood that that constitutes a method that is relatively expensive, both in terms of material resources and in terms of the time needed for achieving a thermal barrier coating that is satisfactory compared with the specifications.